A storage device with automatic-switching function is disclosed. When the storage device is coupled to a USB interface, the power provided by the USB interface turns the USB/SATA converter on to convert data from the USB interface into SATA format and transmit to a hard disk. When the storage device is coupled to a SATA interface instead of the USB interface, the power provided by the USB interface does not turn the USB/SATA converter on to convert data. In this way, the data from the SATA interface directly transmit to the hard disk.